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   Re: [xml-dev] A thought on XLink in PIs for processing directives

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Actually, I've been wondering why W3C XML Schema doesn't use processing 
instructions instead of the xsi:schemaLocation attribute. Multiple PI's 
seem to be better than a space-seperated list of URI's. Can somebody 
point me to the rationale?


Michael Brennan wrote:

> While tinkering with XLink and RDDL, I had an idea that I thought I'd toss
> out on this list. I'd be interested to know if there have been similar
> proposals in the past.
> A number of specs put forward specialized mechanisms for associating
> external resources with a document for a specific purpose. For example, XML
> Schema gives us the "xsi:schemaLocation" attribute, XML Catalog gives us the
> "oasis-xml-catalog" PI, and for associating stylesheets, we have the
> "xml-stylesheet" PI. It seems like these could be unified into one
> generalized mechanism by building upon XLink and RDDL constructs. You
> couldn't use XLink syntax directly since these examples all must be declared
> before the document element or on the document element itself. But it seems
> to me that one could designate a generalized PI that mimics XLink simple
> links that could achieve the desired result.
> For example, the attribute "xsi:schemaLocation='http://foo/bar foobar.xsd
> http://yadda/yadda yadda.xsd'" could become:
>   <?xml-link role="http://www.w3.org/2001/XMLSchema";
> arcrole="http://foo/bar"; href="foobar.xsd"?>
>   <?xml-link role="http://www.w3.org/2001/XMLSchema";
> arcrole="http://yadda/yadda"; href="yadda.xsd"?>
> The PI <?xml-stylesheet type="text/css" href="style.css"?> could become:
>   <?xml-link
> role="http://www.isi.edu/in-notes/iana/assignments/media-types/text/css";
>           href="style.css"?>
> The PI <?oasis-xml-catalog catalog="http://example.com/catalog.xml";?> could
> become:
>   <?xml-link role="urn:oasis:names:tc:entity:xmlns:xml:catalog" 
>              href="http://example.com/catalog.xml";?>
> This seems to me better than the current situation for the following
> reasons:
> * Provides a consistent, extensible mechanism for these sorts of processing
> directives.
> * Leverages namespaces and URIs for identifying the intent and purpose of a
> directive.
> * Does not clutter up the document's infoset with processing directives as
> attributes do.
> Is this a reasonable idea or am I off my rocker on this? Have there been
> similar such proposals?
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